A fiber optic cable typically includes: (1) a fiber or fibers; (2) a buffer or buffers that surrounds the fiber or fibers; (3) a strength layer that surrounds the buffer or buffers; and (4) an outer jacket. Optical fibers function to carry optical signals. A typical optical fiber includes an inner core surrounded by a cladding that is covered by a coating. Buffers typically function to surround and protect coated optical fibers. Strength layers add mechanical strength to fiber optic cables to protect the internal optical fibers against stresses applied to the cables during installation and thereafter. Example strength layers include aramid yarn, steel and epoxy reinforced glass roving. Outer jackets provide protection against damage caused by crushing, abrasions, and other physical damage. Outer jackets also provide protection against chemical damage (e.g., ozone, alkali, acids).
It is well known that micro-bending of an optical fiber within a cable will negatively affect optical performance. Shrinkage of the outer jacket of a fiber optic cable can cause axial stress to be applied to the optical fiber, which causes micro-bending of the optical fiber. One cause of jacket shrinkage is thermal contraction caused by decreases in temperature. Another source of shrinkage is post-extrusion shrinkage.
Shrinkage caused by thermal contraction is typically only temporary. The amount of thermal expansion/contraction is dependent upon the coefficients of thermal expansion of the materials involved. In a typical fiber optic cable, the jacket has a higher coefficient of thermal expansion than the fiber. Thus, when the temperature drops due to normal environmental temperature cycling, the jacket may shrink more than the fiber causing stresses to be applied to the fiber. These stresses are typically only temporary since the jacket will expand back to its original size when the temperature returns to normal.
Post-extrusion shrinkage is a by-product of the extrusion process used to manufacture fiber optic cables. Generally, to make a fiber optic cable, an optical fiber is passed through an extrusion die and molten plastic material is extruded about the exterior of the fiber. As the molten plastic exits the extrusion die, the plastic is elongated in the direction of flow and then passed through a cooling bath where the elongated shape of the plastic is set. However, after the shape has been set, the plastic material continues to have “memory” of the pre-elongated shape. Thus, if the cable is later heated, the plastic material will gravitate towards its pre-elongated shape thereby causing post-extrusion axial shrinkage of the cable jacket. As indicated above, cable jacket shrinkage can cause micro-bending of the optical fiber thereby degrading signal quality. Unlike shrinkage caused by thermal contraction, post-extrusion shrinkage of the type described above is permanent.
Post-extrusion shrinkage is a significant problem in the area of optical fiber connectorization. When a connector is mounted to the end of a fiber optic cable, a heat cure epoxy is often used to secure the connector to the jacket and strength layer. When the epoxy is heated during the cure cycle, the cable jacket is also heated thereby causing permanent post-extrusion shrinkage. Post-extrusion shrinkage can also be caused after installation by environmental temperature variations.